Addon script not working
The bl_idname is not allowed to contain more than one dot. The error message is:
Traceback (most recent call last):
  File &quot;\Text&quot;, line 52, in &lt;module&gt;
  File &quot;\Text&quot;, line 44, in register
RuntimeError: Error: Registering operator class: 'OBJECT_OT_applymodifiersinselected', invalid bl_idname 'object.apply.modifiersinselected', must contain 1 '.' character

This message can be seen when starting Blender from the terminal on Linux, macOS and Windows. On Windows you can also use Window &gt; Toggle System Console.
In order to fix this pick a different bl_idname, for example:
 bl_idname = &quot;object.apply_modifiers_in_selected&quot;